1. Field of the Invention
The present invention relates to mobile telephony and, more specifically, to a two-channel CDMA mobile telephone system.
2. Background of the Invention
Mobile telephones employ several different communication standards. One contemporary mobile device standard is referred to as “3GPP2,” which employs “code division multiple access (CDMA) technology. CDMA is a signal processing technology employing spread-spectrum communications. CDMA does not assign a specific frequency to each communication, but rather uses the full available spectrum with communications encoded in digital sequences. Some CDMA-capable mobile devices employ the ability to communicate simultaneously via two separate channels via two separate transceivers.
The emergency dispatch system allows telephone users to contact an emergency dispatch center by dialing an emergency number. (The standard emergency number in the United States is “9-1-1.”) Once a 9-1-1 call is received, the emergency dispatch center locates the caller and dispatches the appropriate responder (e.g., police department or fire department) to the location of the call. Locating the caller is easy when the call originates from a fixed land line, as the calling telephone is always at fixed location stored in a database that is accessible to the emergency dispatch center. However, locating the caller is more difficult when the call originates from mobile telephone, as the mobile telephone is generally not at a fixed location. One method of locating a mobile telephone is to triangulate the general location of the mobile telephone by calculating the signal return time from several different cellular telephone towers. This method does not provide a precise result due to inherent limitations in the distance measurement capabilities of the cellular system.
Some cellular telephones include a global positioning system (GPS) circuit that is used to make a precise determination of location using GPS satellite information. Typically, when a GPS-capable system initiates a call, it first triangulates the general location from the cellular towers that it is communicating with. Then it receives information of a cellular network position determination entity (PDE) regarding the nearest GPS satellites from which it may receive position data. The system then acquires GPS location data from the satellites and transmits the data to the PDE, which calculates and stores the precise location of the mobile device.
Typically, this is done by suspending transmission on the communications channel of the communications channel while the location is being determined. In some systems, the mobile device switches between voice communication and satellite communication in alternating two second blocks. This results in a poor experience for the parties participating in a conversation. A preferred response quality (PRQ) period is set by the PDE and corresponds to the amount of time that the mobile device has to acquire GPS location data and transmit the data to the PDE. A longer PRQ results in a more precise location determination. However, a longer PRQ also results in degraded communications signal quality, as transmission on communications signal bandwidth is suspended during the GPS data acquisition process. To limit communications degradation, the PRQ is currently limited to 16 seconds in most standards. However, such a limited PRQ may not be sufficient to provide a precise location of the mobile device.
In GPS sessions on 3GPP2 networks, most phones use time-multiplexed GPS searches to look for satellites. This leaves gaps in the audio and provides a poor user experience. Because of this, one mobile service provider has set the timeout for the mobile to report GPS measurements to 16 seconds. In the past, this setting was 32 seconds or longer, but it was reduced to minimize user complaints about audio gaps. Some new CDMA phones contain two receive chains, and allow simultaneous searches of GPS and CDMA. However, the network element responsible for sending the GPS search parameters to the mobile device typically does not know the mobile's capabilities so it can tune the parameters accordingly.
Therefore, there is a need for a mobile communications system that allows an extended PRQ without degrading the quality of communications transmitted over the system.